Pump NPSH available

NPSH available and margin for open tanks and closed vessels — with water vapour pressure from steam tables and suction-line friction.

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Pump performance against the maker's curve, NPSH with full suction-line losses, pipe sizing and pressure drop, and control valve sizing — with print-ready reports, any units and visible formulas.

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How this calculator works

NPSH available = pressure head on the liquid surface − vapour pressure head + liquid level above the pump − suction line friction. For water, density, viscosity and vapour pressure come from steam tables (vapour pressure by the IAPWS-IF97 equation). For an open tank, the air pressure is worked out from the site altitude. Friction is calculated by Darcy-Weisbach with the Swamee-Jain friction factor, or you can enter it directly.

The example is 80 °C water pumped at 120 m³/h from an open tank at 300 m altitude, with the level 3 m above the pump and 25 m equivalent length of 150 mm pipe. NPSH available is 7.8 m; with 4.5 m required, the margin is 3.3 m and the ratio 1.73.

NPSH required is not a safe limit. The maker's NPSHr is the point where the pump has already lost 3 % of its head to cavitation. Allow a margin — a ratio of at least 1.3 and 1 m (3 ft) is a common screening target, and hot water, high suction energy and long running hours need more. Hydraulic Institute guidance (ANSI/HI 9.6.1) and the pump maker set the margin for your service.

Limits: single liquid, no dissolved gas; for hydrocarbon mixtures use the bubble-point pressure as the vapour pressure. References: Perry's Chemical Engineers' Handbook §6 and §10; Hydraulic Institute ANSI/HI 9.6.1; IAPWS-IF97; Swamee and Jain (1976).

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